The invention relates to an optical structure, more particularly to an optical structure comprising an optical resonator that can for instance be a circular Bragg resonator, with an improved coupling to a waveguide.
Various photonic functionalities can be implemented on the basis of an optical resonator. Prominent examples thereof are spectral filters, light sources such as lasers, electro-optic modulators (EOMs), and all-optical switches.
For many applications, e.g. chip to chip or on-chip optical interconnects, a higher physical integration density of such optical resonator-based functionalities is desirable. Integrated micro-resonators can be very useful. Such integrated optical structures typically make use of waveguides coupled to the micro-resonators. Therein the coupling efficiency is a critical parameter.
In M. A. Kaliteevski et al., J. of Modern Optics 46, 875 (1999) “Bragg reflectors for cylindrical waves” the electromagnetic field in a dielectric structure with circular cylindrical symmetry is described.
In Ochoa et al., Phys. Rev. B 61, 4806 (2000) “Diffraction of cylindrical Bragg reflectors surrounding an in-plane semiconductor microcavity” a model is described for finding the resonant modes of a microcavity bound by a circular Bragg reflector.
In U.S. Pat. No. 4,743,083 devices for controlling and guiding waves, such as electromagnetic waves, acoustic waves, and the like are disclosed which include at least one wave-transmitting medium and a diffraction grating, associated with the transmitting medium, for coupling wave energy into or out of the waves propagating in the grating. The device may take the form of a “sandwich”-type or thin film waveguide, in which case the grating medium has at least one curvi-planar boundary, or it may take the form of a “rod”-shaped or cylindrical waveguide, in which case the grating medium has a substantially cylindrical boundary. A wave propagating in the grating medium is coupled to an unguided free-space type wave mode which is perpendicular to the grating medium. This is achieved by at least part of the grating being of second order in terms of its periodicity. For example, in the case of a planar medium and a circular grating in that medium this corresponds to coupling from the plane of the grating to an unguided wave propagating in a direction perpendicular to that plane.
US 2005/0135453 is directed to a grating-outcoupled microcavity disk resonator that has whispering gallery modes existing in a nearly circular resonator. Light is outcoupled by providing a grating region in the plane of the grating-outcoupled microcavity disk resonator. The grating region provides an outcoupling mechanism that symmetrically interacts with the clockwise and counterclockwise whispering gallery modes, thereby making the resonator capable of surface emission.
J. Scheuer et al., IEEE J. Sel. Top. Quant. Electron. 11, 476 (2005) “InGaASP Annular Bragg Lasers: Theory, Applications, and Modal Properties” discusses a class of circular resonators, based on a radial defect surrounded by Bragg reflectors.
In M. Toda, IEEE J. of Quantum Electronics 26, 473 (1990) “Single-Mode Behavior of a Circular Grating for Potential Disk-Shaped DFB Lasers” a coaxial circular grating resonator for a DFB laser application is described wherein the periodicity and the position of the grating are so chosen that all of the reflections from each refractive index step are superimposed in phase so as to be consistent with the resonant behavior of the fundamental mode wave.
In X. H. Zheng et al., IEEE J. of Lightwave Technology 8, 1509 (1990) “Mode Coupling in Circular-Cylindrical Systems and Its Application to Fingerprint Resonators” a circular resonator with implanted taper-like structures is described. The taper structures are used for realizing energy input and output, similar to a small probe in a classic microwave resonator.
C. Wu et al., Electronics Letters 27, 1819 (1991) “Optically pumped surface-emitting DFB GaINAsP/InP Lasers with circular grating” examines the lasing action in DFB lasers with circular gratings.
In “High-finesse disk microcavity based on a circular Bragg reflector”, D. Labilloy et al., Appl. Phys. Lett. 73, 1314 (1998) it is described how disk-shaped microcavities are bounded by reflectors made of circular concentric deep-etched trenches.